Over the recent years, a development has been accelerated of a ferroelectric memory (FeRAM) which retains information on a ferroelectric capacitor by utilizing polarizing inversion of a ferroelectric substance. The ferroelectric memory is a nonvolatile memory from which the retained information does not disappear even when switching off a power source, and is focused especially in terms of enabling high integration, high durability and low power consumption to be actualized.
A material of a ferroelectric film constituting the ferroelectric capacitor involves using mainly a ferroelectric oxide having a perosvkite crystalline structure such as a PZT(Pb(Zr, Ti)O3) film and an SBT (SrBi2Ta2O9) film on the order of 10μ-30 μC/cm2, which has a large residual polarization quantity. It has hitherto been known that in this type of ferroelectric film, a characteristic of the ferroelectric substance is deteriorated due to a water content permeating from outside via an inter-layer insulating film such as a silicon oxide film having high affinity with the water. To be specific, the permeated water content is dissolved into hydrogen and oxygen in a high-temperature process when the inter-layer insulating film and a metal wiring layer are grown, resulting in the permeation into the ferroelectric film. Thereupon, the hydrogen reacts to the oxygen of the ferroelectric film, and an oxygen defect occurs in the ferroelectric film, with the result that crystallinity decreases. Further, a long-term use of the ferroelectric memory causes the same phenomenon. As a result, deterioration of performance of the ferroelectric capacitor arises such as decreases in residual polarization quantity and in dielectric constant of the ferroelectric film. Moreover, without being limited to the ferroelectric capacitor, the performance of a transistor etc might deteriorate.
To cope with such deterioration, aluminum oxide (Al2O3), which prevents the permeation of the hydrogen and the water content, has hitherto been employed. For example, the aluminum oxide is formed so as to embrace the ferroelectric capacitor and protects the ferroelectric capacitor so that the hydrogen/water content do not permeate an interior of the ferroelectric substance. Further, the aluminum oxide is formed upwardly of (just above) a first wiring and is utilized as a barrier film that prevents further permeation, into a lower layer, of the water and the hydrogen permeating from an upper portion of a semiconductor element. Herein, the first wiring represents a lowermost layer of wiring layers, which is, i.e., the wiring layer closest to a transistor or a layer of the ferroelectric capacitor.
Further, what is known about the ferroelectric capacitor is that the characteristic of the ferroelectric substance is deteriorated when high-temperature heat is applied thereto in a multi-layer process for a long period of time. The process of applying the high-temperature heat for the long time is exemplified by a process of forming polyimide becoming a final passivation film. This process has hitherto involved executing a treatment, e.g., at a temperature of 310° C.-350° C. for a period of baking time of 60 min. The characteristic of the ferroelectric substance, however, deteriorates because of being affected by the heat, the hydrogen and the water content generated when forming the polyimide. Therefore, such a scheme was proposed that a flat aluminum oxide film (aluminum film) is disposed between the uppermost wiring layer including a pad electrode and a wiring layer existing under this uppermost wiring layer. The aluminum film blocks the permeation of the hydrogen and further the water content generated from the polyimide into a semiconductor element.
[Patent document 1] Japanese Laid-open Patent Publication No. 2006-66906
[Patent document 2] Japanese Patent No. 3029316